Project Summary and Abstract Hypothyroidism affects 10-15 million Americans, mostly women, and between 300-400 million worldwide. For about 90 years since the late 19th century, hypothyroidism was treated with capsules of desiccated animal thyroid. They contain both hormones produced by the thyroid gland, the inactive precursor T4 and the active hormone T3. In 1970, things changed with the observation that T4 is secreted by the thyroid gland and converted to T3 throughout the body via D1 (15%) or D2 (85%). This led to treatment with L-T4 alone, a decision that assumed D1 and D2 could generate sufficient T3, including the small amounts of T3 secreted directly from the thyroid. Also in the 1970s, the RIA for TSH was developed. We learned that D2 is critical for TSH feedback: low levels of T4 decrease D2-generated T3 in the hypothalamus and pituitary, elevating TSH. Hypothyroid patients are started on L-T4 and the dose is adjusted up until TSH levels are back to normal. Therefore, L-T4 monotherapy at doses that normalize serum TSH became the standard of care in the early 1980s; L-T4 became the #1 prescribed medication in the US, with 21.5 million prescriptions/month in 2014-15. However, there are doubts that either one, or both deiodinases can maintain optimal levels of T3 in the absence of a functional thyroid gland. We looked at 9,981 participants in the US NHANES who had normal TSH levels; among whom 469 were treated with L-T4. When comparing with matched healthy controls by age, sex, race and serum TSH, we saw that those on L-T4 were ~10 pounds heavier despite consuming fewer calories; they were also more likely to be on anti-depressants, statins or beta-blocker. It is fascinating that D2 is at the epicenter of this; hypothyroid patients are given only L-T4 and must rely on D2 to generate >85% of all T3 in their bodies. In this proposal, we will investigate not only the molecular basis of how L-T4 therapy affects D2 regulation and T3 production but also its broad implication for metabolism. We will focus on very significant clinical questions, e.g. what is the relevance of Thr92Ala-D2 polymorphism? We now have a mouse model to address this question.